This invention relates to an improved process for the conversion of aromatic hydrocarbons. More specifically, the present invention concerns disproportionation and transalkylation of aromatic hydrocarbons to obtain xylenes.
The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is para-xylene, the principal feedstock for polyester which continues to enjoy a high growth rate from large base demand. Orthoxylene is used to produce phthalic anhydride, which has high-volume but mature markets. Meta-xylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but usually is considered a less-desirable component of C8 aromatics.
Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Often toluene is dealkylated to produce benzene or disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered.
A current objective of many aromatics complexes is to increase the yield of xylenes and to de-emphasize benzene production. Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. Benzene produced from transalkylation processes often is not sufficiently pure to be competitive in the market. A higher yield of xylenes at the expense of benzene thus is a favorable objective, and processes to transalkylate C9 aromatics and toluene have been commercialized to obtain high xylene yields.
Such transalkylation processes have been disclosed in the patent literature for some time. U.S. Pat. No. 3,597,490 (Otani et al.) teaches toluene conversion along with 0.5-50 mol percent C9 aromatics using a solid acid catalyst. The reaction is carried out at a temperature of 300xc2x0 to 650xc2x0 C. Separation and recycle of xylene isomers also is disclosed.
U.S. Pat. No. 5,177,286 (Hagen et al.) teaches production of p-alkyltoluene via transmethylation with a feed which may comprise toluene and a variety of polymethylbenzenes. The catalyst comprises a Lewis acid and/or Bronsted acid that is more acidic than ferric chloride and at least as acidic as ferric bromide. Operating conditions comprise a temperature of from about xe2x88x9240xc2x0 to 80xc2x0 C.
Conventional technology for transalkylation is based on catalysts comprising zeolitic aluminosilicates, believed to be effected via a strong Brxc3x6nsted acid. A lower-energy path would provide potential for greater selectivity and improved economics.
It is an object of the present invention to provide an improved process for the transalkylation of aromatic hydrocarbons. A specific objective is obtain a high yield of xylenes by transalkylation of toluene and, optionally, higher aromatics.
This invention is based on the discovery that transalkylation of toluene and C9 aromatics using a sulfated zirconia catalyst shows favorable results at mild conditions.
A broad embodiment of the present invention is directed to a process for the transalkylation of a toluene-containing feedstock to obtain a product comprising para-xylene using a catalyst comprising one or more sulfated oxides and hydroxides of elements of Group IVB (IUPAC 4) of the Periodic Table. The catalyst preferably comprises a refractory inorganic-oxide binder, especially alumina.
The feedstock preferably comprises C9 aromatics which are transalkylated along with toluene to provide a higher yield of C8 aromatics. The transalkylation preferably is effected in the liquid phase at relatively mild conditions, comprising a temperature within the range of about 110xc2x0 to 250xc2x0 C.
Mixed C8 aromatics recovered from the transalkylation effluent optionally is sent a xylene-separation zone; preferably, para-xylene is recovered by adsorption.
These as well as other objects and embodiments will become apparent from the detailed description of the invention.